Boffins map surface of Titan by accident

Every planetary landing mission that flies from now on will effectively gain an new sensor set without a single extra piece of equipment, thanks to a chance discovery by scientists working on data from the Huygens lander.

When Huygens unexpectedly survived the impact of its landing on Saturn's moon, Titan, the radio signal it was sending to its mothership, Cassini, was reflected off the surface of the moon. This unexpected reflection interfered with the main signal.

The ESA team has been able to use the pattern of interference to learn more about the surface of the moon. Now, they say that by subtly altering the properties of a radio beam, a lander on a future mission could be tweaked to send scientists useful information about the chemical composition of the surface, for instance.

Miguel Pérez-Ayúcar a member of the Huygens Team at ESA’s European Space Research and Technology Centre (ESTEC) in The Netherlands commented: “Huygens had not been designed to necessarily survive impact so we had never thought about what the signal would look like from the surface,” says Pérez.

He jokes that the team initially suggested the signal was being caused by aliens dragging the lander across the surface.

However, after a more serious investigation, Pérez and his team worked out what was going on.

Cassini was not in orbit around Titan, but was flying by on its way to Saturn. As it travelled away from the Huygens landing site, the angle between it and the lander changed. This altered the way in which the interference between the reflected and direct beams was detected, causing variation in the signal's power.

Pérez built a computer simulation that not only reproduced the effect the team had observed, but revealed that it was sensitive enough to pick out variations caused by individual pebbles on the moon's surface.

In the 71 minutes the lander spent on the surface before Cassini flew out of range, this reflection interference sent back information about a two kilometre stretch of the moon's surface.

The team was able to conclude that it was quite flat, but strew with pebbles of between five and 10cm in diameter.

The serendipitous discovery will be everyone's gain.

"This experience can be inherited by any future lander," said Pérez, "All that will be needed is a few refinements and it will become a powerful technique." ®